Leading male in a commercial (5)
I believe the answer is:
ahead
'leading' is the definition.
(to lead is to go ahead)
'male in a commercial' is the wordplay.
'male' becomes 'he' (he refers to a man).
'in' means one lot of letters goes inside another.
'commercial' becomes 'ad' (abbreviation for advertisement).
'a'+'ad'='aad'
'he' inserted into 'aad' is 'AHEAD'.
(Other definitions for ahead that I've seen before include "In front as in race" , "Forward - in front" , "in van" , "Winning" , "advanced" .)